Electrostatic Assembly Preparation of High-Toughness Zirconium Diboride-Based Ceramic Composites with Enhanced Thermal Shock Resistance Performance

The central problem of using ceramic as a structural material is its brittleness, which associated with rigid covalent or ionic bonds. Whiskers or fibers of strong ceramics such as silicon carbide (SiC) or silicon nitride (Si₃N₄) are widely embedded in a ceramic matrix to improve the strength and toughness. The incorporation of these insulating fillers can impede the thermal flow in ceramic matrix, thus decrease its thermal shock resistance that is required in some practical applications. Here we demonstrate that the toughness and thermal shock resistance of zirconium diboride (ZrB₂)/SiC composites can be improved simultaneously by introducing graphene into composites via electrostatic assembly and subsequent sintering treatment. The incorporated graphene creates weak interfaces of grain boundaries (GBs) and optimal thermal conductance paths inside composites. In comparison to pristine ZrB₂–SiC composites, the toughness of (2.0%) ZrB₂–SiC/graphene composites exhibited a 61% increasing (from 4.3 to 6.93 MPa·m^1/2) after spark plasma sintering (SPS); the retained strength after thermal shock increased as high as 74.8% at 400 °C and 304.4% at 500 °C. Present work presents an important guideline for producing high-toughness ceramic-based composites with enhanced thermal shock properties

Sonochemical Transformation of Epoxy–Amine Thermoset into Soluble and Reusable Polymers

The degradation and reuse of epoxy thermosets have significant impact on the environments. We report that an epoxy–amine thermoset embedded with Diels–Alder (DA) bonds was transformed into soluble polymers via sonochemistry under mild temperature (ca. 20 °C) for the first time. Sonication could effectively induce the position-oriented cleavage of DA bonds (i.e., retro-DA) of the fully swelled epoxy thermoset in dimethyl sulfoxide (DMSO), leading to the soluble polymers. Of importance, such sonochemical process could be regulated on demand via switching on-and-off of the sonication. The obtained soluble polymers could be recured to form epoxy–amine thermosets via DA reaction. This sonochemical method might provide an unprecedented and efficient way to the controlled degradation and recycling of the epoxy thermosets containing the dynamic covalent bonds likes DA groups

Thermosensitive Ionic Microgels via Surfactant-Free Emulsion Copolymerization and in Situ Quaternization Cross-Linking

A type of thermosensitive ionic microgel was successfully prepared via the simultaneous quaternized cross-linking reaction during the surfactant-free emulsion copolymerization of <i>N</i>-isopropylacrylamide (NIPAm) as the main monomer and 1-vinylimidazole or 4-vinylpyridine as the comonomer. 1,4-Dibromobutane and 1,6-dibromohexane were used as the halogenated compounds to quaternize the tertiary amine in the comonomer, leading to the formation of a cross-linking network and thermosensitive ionic microgels. The sizes, morphologies, and properties of the obtained ionic microgels were systematically investigated by using transmission electron microscopy (TEM), dynamic and static light scattering (DLS and SLS), electrophoretic light scattering (ELS), thermogravimetric analyses (TGA), and UV–visible spectroscopy. The obtained ionic microgels were spherical in shape with narrow size distribution. These ionic microgels exhibited thermosensitive behavior and a unique feature of poly­(ionic liquid) in aqueous solutions, of which the counteranions of the microgels could be changed by anion exchange reaction with BF₄K or lithium trifluoromethyl sulfonate (PFM-Li). After the anion exchange reaction, the ionic microgels were stable in aqueous solution and could be well dispersed in the solvents with different polarities, depending on the type of counteranion. The sizes and thermosensitive behavior of the ionic microgels could be well tuned by controlling the quaternization extent, the type of comonomer, halogenated compounds, and counteranions. The ionic microgels showed superior swelling properties in aqueous solution. Furthermore, these ionic microgels also showed capabilities to encapsulate and release the anionic dyes, like methyl orange, in aqueous solutions

In Situ Growth of Core–Sheath Heterostructural SiC Nanowire Arrays on Carbon Fibers and Enhanced Electromagnetic Wave Absorption Performance

Large-scale core–sheath heterostructural SiC nanowires were facilely grown on the surface of carbon fibers using a one-step chemical vapor infiltration process. The as-synthesized SiC nanowires consist of single crystalline SiC cores with a diameter of ∼30 nm and polycrystalline SiC sheaths with an average thickness of ∼60 nm. The formation mechanisms of core–sheath heterostructural SiC nanowires (SiC_nws) were discussed in detail. The SiC_nws-CF shows strong electromagnetic (EM) wave absorption performance with a maximum reflection loss value of −45.98 dB at 4.4 GHz. Moreover, being coated with conductive polymer polypyrrole (PPy) by a simple chemical polymerization method, the SiC_nws-CF/PPy nanocomposites exhibited superior EM absorption abilities with maximum RL value of −50.19 dB at 14.2 GHz and the effective bandwidth of 6.2 GHz. The SiC_nws-CF/PPy nanocomposites in this study are very promising as absorber materials with strong electromagnetic wave absorption performance

S, N Dual-Doped Graphene-like Carbon Nanosheets as Efficient Oxygen Reduction Reaction Electrocatalysts

Replacement of rare and precious metal catalysts with low-cost and earth-abundant ones is currently among the major goals of sustainable chemistry. Herein, we report the synthesis of S, N dual-doped graphene-like carbon nanosheets via a simple pyrolysis of a mixture of melamine and dibenzyl sulfide as efficient metal-free electrocatalysts for oxygen reduction reaction (ORR). The S, N dual-doped graphene-like carbon nanosheets show enhanced activity toward ORR as compared with mono-doped counterparts, and excellent durability in contrast to the conventional Pt/C electrocatalyst in both alkaline and acidic media. A high content of graphitic-N and pyridinic-N is necessary for ORR electrocatalysis in the graphene-like carbon nanosheets, but an appropriate amount of S atoms further contributes to the improvement of ORR activity. Superior ORR performance from the as-prepared S, N dual-doped graphene-like carbon nanosheets implies great promises in practical applications in energy devices

Improving Electrocatalysts for Oxygen Evolution Using Ni_<i>x</i>Fe_3–<i>x</i>O₄/Ni Hybrid Nanostructures Formed by Solvothermal Synthesis

Spinel-type oxides have been found to be active electrocatalysts for OER. However, their semiconductor character severely limits their catalytic performance. Herein, we report a facile solvothermal pathway for the synthesis of spinel-type Ni_<i>x</i>Fe_3–<i>x</i>O₄ oxides/Ni metal nanocomposites. The good electrical contact between the metal and semiconductor oxide interface and well-tuned compositions of Ni_<i>x</i>Fe_3–<i>x</i>O₄ spinel oxides are crucial to achieve better OER performance. Specifically, the Ni_<i>x</i>Fe_3–<i>x</i>O₄/Ni nanocomposite sample prepared from a metal precursor ratio of <i>y</i> = 0.15 [<i>y</i> = Fe/(Fe + Ni)] that results in an <i>x</i> value of about 0.36 exhibits catalytic activity with an overpotential of 225 mV to achieve an electrocatalytic current density of <i>j</i> = 10 mA cm^–2 and a Tafel slope of 44 mV dec^–1 in alkaline electrolyte. This study not only provides new perspectives to designing nanocomposite catalysts for OER but also opens a promising avenue for further enhancing electrocatalytic performance via interface and composition engineering

Carbon Nanofiber Arrays Grown on Three-Dimensional Carbon Fiber Architecture Substrate and Enhanced Interface Performance of Carbon Fiber and Zirconium Carbide Coating

Carbon nanofibers (CNFs) were grown around the carbon fiber architecture through a plasma enhanced chemical vapor deposition method to enhance the interface performance between CF architecture substrate and ZrC preceramic matrix. The synthesized 3D CF hierarchical architectures (CNFs-CF) are coated with zirconium carbide (ZrC) ceramic to enhance their antioxidant property and high temperature resistance. The composition and the crystalline phase structure of the composite were detected with the X-ray photoelectron spectroscopy and X-ray diffraction. The results of scanning electron microscopy show that, the as-prepared CNFs and consistent ZrC ceramic coating are uniformly covered on the surface of carbon fiber architecture substrate. The ZrC ceramic products with excellent crystallinity were got from the pyrolysis of preceramic polymer at 1600 °C in inert atmosphere. Comparing with the untreated CF, the loading of ZrC ceramics around the CNFs-CF architecture surface are significantly increased. The thermal stability and mechanical property of CNFs-CF/ZrC nanocomposites have been promoted obviously compared with the CF/ZrC ceramic nanocomposite. The prepared CNFs-CF/ZrC ceramic nanocomposite is one of the potential candidate materials for the thermal protection application

Ordered Silica Nanoparticles Grown on a Three-Dimensional Carbon Fiber Architecture Substrate with Siliconborocarbonitride Ceramic as a Thermal Barrier Coating

Hierarchical structure consisting of ordered silica nanoparticles grown onto carbon fiber (CF) has been fabricated to improve the interfacial properties between the CFs and polymer matrix. To improve the reactivity of CFs, their surface was modified using poly­(1,4-phenylene diisocyanate) (PPDI) via in situ polymerization, which also resulted in the distribution of numerous isocyanate groups on the surface of CFs. Silica nanoparticles were modified on the interface of CF-PPDI by chemical grafting method. The microstructure, chemical composition, and interfacial properties of CFs with ordered silica nanoparticles were comprehensively investigated by scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. Results indicated an obvious increase in the interfacial shear strength, compared to that of CF precursor, which was attributed to silica nanoparticles interacting with the epoxy resin. Furthermore, siliconborocarbonitride (SiBCN) ceramic was used as thermal barrier coating to enhance 3D CF architecture substrate antioxidant and ablation properties. Thermogravimetric results show that the thermal stability of the CF with SiBCN ceramic layer has a marked increase at high temperature

Direct Transformation from Graphitic C₃N₄ to Nitrogen-Doped Graphene: An Efficient Metal-Free Electrocatalyst for Oxygen Reduction Reaction

Carbon-based nanomaterials provide an attractive perspective to replace precious Pt-based electrocatalysts for oxygen reduction reaction (ORR) to enhance the practical applications of fuel cells. Herein, we demonstrate a one-pot direct transformation from graphitic-phase C₃N₄ (g-C₃N₄) to nitrogen-doped graphene. g-C₃N₄, containing only C and N elements, acts as a self-sacrificing template to construct the framework of nitrogen-doped graphene. The relative contents of graphitic and pyridinic-N can be well-tuned by the controlled annealing process. The resulting nitrogen-doped graphene materials show excellent electrocatalytic activity toward ORR, and much enhanced durability and tolerance to methanol in contrast to the conventional Pt/C electrocatalyst in alkaline medium. It is determined that a higher content of N does not necessarily lead to enhanced electrocatalytic activity; rather, at a relatively low N content and a high ratio of graphitic-N/pyridinic-N, the nitrogen-doped graphene obtained by annealing at 900 °C (NGA900) provides the most promising activity for ORR. This study may provide further useful insights on the nature of ORR catalysis of carbon-based materials

Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets

Molybdenum disulfide (MoS₂) is a promising nonprecious catalyst for the hydrogen evolution reaction (HER) that has been extensively studied due to its excellent performance, but the lack of understanding of the factors that impact its catalytic activity hinders further design and enhancement of MoS₂-based electrocatalysts. Here, by using novel porous (holey) metallic 1T phase MoS₂ nanosheets synthesized by a liquid-ammonia-assisted lithiation route, we systematically investigated the contributions of crystal structure (phase), edges, and sulfur vacancies (S-vacancies) to the catalytic activity toward HER from five representative MoS₂ nanosheet samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated porous 2H phase. Superior HER catalytic activity was achieved in the porous 1T phase MoS₂ nanosheets that have even more edges and S-vacancies than conventional 1T phase MoS₂. A comparative study revealed that the phase serves as the key role in determining the HER performance, as 1T phase MoS₂ always outperforms the corresponding 2H phase MoS₂ samples, and that both edges and S-vacancies also contribute significantly to the catalytic activity in porous MoS₂ samples. Then, using combined defect characterization techniques of electron spin resonance spectroscopy and positron annihilation lifetime spectroscopy to quantify the S-vacancies, the contributions of each factor were individually elucidated. This study presents new insights and opens up new avenues for designing electrocatalysts based on MoS₂ or other layered materials with enhanced HER performance